The present invention is further detailed in Japanese Patent Application No. 5-172145 filed Jun. 19, 1993, for which the present application claims priority and the entire content of which is incorporated herein by reference.
1. Field of the Invention
The present invention provides a process for molding a semi-rigid integral skin foam, wherein the semi-rigid integral skin foam comprises an integral skin region having a low degree of foaming and a core region having a high degree of foaming. The present invention is suitable for reaction injection molding (RIM) of a molded product having an integral skin foam, such as a cover or pad for a steering wheel of a motor vehicle, an instrument panel, a console box lid, a glove box lid, a headrest, an armrest, or an air spoiler.
2. Description of Related Art
A semi-rigid polyurethane foam containing an integral skin region having a low degree of foaming on its surface and a core region having a high degree of foaming at its center is generally referred to as an integral skin foam (ISF). It has been usual to mold an ISF by injecting, into a mold cavity, a polyurethane material of a polyol component and an isocyanate component and containing freon, and by vaporizing and expanding freon by the heat generated by a urethane-forming reaction.
At the center of the polyurethane material, the urethane-forming reaction and the vaporization of freon take place simultaneously, sustaining a myriad of freon bubbles. This results in the formation of a core having a high degree of foaming. The polyurethane material close to the wall surface of the mold cavity undergoes the urethane-forming reaction more slowly than that at the center, because the reaction heat dissipates through the wall surface of the mold cavity. One skilled in the art will appreciated that the slower the reaction is characterized by a lower reaction heat and, consequently, a reduced number of bubbles will be formed from a decreased vaporization of freon. The bubbles are compressed or collapsed by the internal foaming pressure. The result is the formation of an integral skin having a low degree of foaming.
The above-described conventional method for producing an ISF by the aid of freon poses the following problems: freon released to the atmosphere from any industrial or commercial process is recognized as a global problem and reductions in its use are being mandated by many countries;, formation of the integral skin by collapsing bubbles at the surface section by the internal foaming pressure allows minute bubbles to inevitably remain in the skin which ultimately impair the appearance of the final foamed product; and overpacking and subsequent discharge of the polyurethane material from a vent is necessary to increase the internal foaming pressure and to promote the mold filling. Ultimately this leads to a great loss of material.
Further, freon foamed processes which include a core in the mold cavity, such as is common in the production of steering wheels, suffer from turbulence induced defects such as pinholes, voids and shortshot, due to entrapped air. This is particularly a problem in the production of steering wheels, in which case a long ring core is placed in the mold cavity. Thus, it is necessary to select an adequate place for gating to minimize turbulence and to form several vents to release entrapped air.
We, the applicant of the present application, have previously filed a patent application covering an invention of a process for molding an ISF without using any freon. According to that invention, a polyurethane material containing substantially no foaming agent is injected into an evacuated mold cavity to be expanded (Japanese Patent Application No. 3-277285). It is desirable to evacuate the mold cavity to 50 Torr or below in order to form an integral skin which should have a firm feel. When the reduced pressure in the mold cavity (or its vacuum degree) exceeds 50 Torr, it has been difficult to mold any ISF including a core having a high degree of foaming and low density, since the polyurethane material contains substantially no foaming agent.
Alternative methods for producing polyurethane foams using reduced pressure are disclosed in the following Japanese Patents, however, these patents do not necessarily disclose or suggest the object, constitution and effect of the present invention.
Japanese Patent Laid-open Nos. 55-63237 and 55-63238 disclose a method for causing a polyurethane material to uniformly expand to fill a mold cavity completely, by evacuating the mold cavity through thin grooves formed in the walls of the mold cavity. However, these patents presumably teach the use of standard foaming agents as only conventional compositions are disclosed.
Japanese Patent Laid-open No. 56-111648 discloses a method for foaming a polyurethane material in an atmosphere of reduced pressure. The patentee claims that this method provides for a reduction in the amount of a foaming agent, such as freon, required. However, this method employs freon as conventionally used. This patent does not provide for the advantageous elimination of freon, as in the present invention.
Japanese Patent Laid-open No. 62-164709 discloses a method for producing a low-density polyurethane foam by expanding a polyurethane material which contains water as a foaming agent in an atmosphere of reduced pressure. Unlike the present invention, this method is not intended to produce an ISF. The formation of the skin is not desirable.
Japanese Patent Laid-open No. 63-268624 discloses a method for producing a polyurethane foam by causing a polyurethane material to capture nitrogen gas as much as 230 vol%, and expanding the material in an atmosphere of reduced pressure. This method is not taught to be useful to produce an ISF.
Japanese Patent Publication No. 64-5528 discloses a method for producing a polyurethane foam by injecting a rigid polyurethane material into an evacuated mold cavity at a packing ratio of about 150-450%, and subsequently expanding the material, however, the polyurethane material contains a foaming agent, such as freon.